In order to address the physiological functions of the TNF/LT subfamily of cytokines we used Cre-loxP technology and have generated, in collaboration with laboratories of K. Pfeffer and L. Rajewsky, a panel of knock-out mice with single (LT-beta) , double (LT-beta-TNF) and complete (triple) deficiency in this locus. This allowed us to reveal distinct and overlapping contributions of TNF and LT signalling into organogenesis and maintenance of lymphoid tissues. We have demonstrated that LT deficiency is associated with enhanced tumor growth and metastasis and with impairment in development and recruitment of NK cells. Cre-loxP technology is also being used in collaboration with laboratories of L. Tessarollo and C. Stewart to generate mice with conditionally targeted TNF and LT genes. In order to identify genes responsible for mutant phenotype in distinct histological compartments and organs in mice with LT, TNF or combined deficiencies, we are searching for genes whose expression is substantially altered in knock- out mice. In particular, by subtraction cloning and by comparative hybridizations to gene arrays we have identified a number of known and novel sequences which are expressed at much lower level in spleens of LT- deficient mice. These genes include novel phospholipase A2, chemokines of lymphoid organs ELC, SLC and BLC, macrophage scavenger receptor, myeloperoxidase and others. Causative role of these genes in development and maintenance of mutant phenotype will be addressed in transgenic studies. Our studies may also reveal relevance of these murine models for human disease, including AIDS. - Gene Targeting, LT, macrophages, TNF, TNF/LT, - Neither Human Subjects nor Human Tissues